1. Field of the Invention
The present invention relates to a dry toner employed in recording methods utilizing electrophotography, electrostatic recording, electrostatic printing, magnetic recording, a toner jet method, or the like, a method for producing the dry toner, and a method for forming an image. The present invention more specifically relates to a dry toner employed in an image forming apparatus which may be used for a copier, a printer, a facsimile, a plotter, or the like, a method for producing the dry toner, and a method for forming an image.
2. Description of the Related Art
Electrophotography providing a fixed image usually involves: forming an electrical latent image on a photosensitive member through various means; subsequently developing the latent image using a toner; transferring a toner image to a transfer material such as paper as appropriate; and fixing the toner image through heating, pressing, heat pressing, or solvent vapor.
A dry toner employed for the electrophotography (hereinafter, referred to as “toner”) usually consists of colored resin fine particles containing a binder resin, a colorant, and waxes as main components. The colored resin fine particles generally have particle diameters of about 6 to 15 μm with respect to number-average particle diameter. A method for producing the toner consisting of such colored resin fine particles generally involves: melt-kneading the binder resin, the waxes, the colorant such as pigments, dyes, and/or magnetic materials, or the like; cooling the kneaded product; pulverizing the cooled product; and then classifying the pulverized product to provide toner particles(this method is called “pulverization process”). However, according to the pulverization process, size reduction of the toner particles to fine particles or shape control of the toner particles not only provokes reduction in productivity, and further there is a limit to actively designing an inner structure of the toner particles. On the other hand, the production of the toner through “polymerization process” has been carried out for overcoming problems of the toner production through pulverization process, and in addition, for further achieving an improvement of toner performance through functionalization of the toner to higher levels. To be specific, the polymerization process is roughly divided into suspension polymerization process and emulsion aggregation process.
Recently, application fields of an image forming apparatus utilizing the electrophotography have rapidly developed with increased wide variety of demands not only as a copier for simply copying an original, but also as a printer which is an output device of a computer and as a personal copy, further as a plain paper facsimile or the like. Further, the functionalization of the copier to higher levels through digitalization has progressed. In particular, miniaturization of an image forming apparatus unit, speeding up, and colorization are significantly progressing, and further, high reliability and high resolution are strongly demanded. For example, a resolution, which was initially 200 to 300 dots per inch (dpi), is now 400 to 1,200 dpi, further increasing to 2,400 dpi. Further, according to full-color image forming, a multi-color image is generally reproduced by: repeating development and transfer of an electrostatic latent image using a magenta toner, a cyan toner, a yellow toner, and a black toner; superimposing toner images of respective colors; and fixing the superimposed images. Such a high resolution and/or full-color image forming apparatus have been designed with simpler components using members with various functions of high levels for satisfying the above demands. Accordingly, the level of the functions demanded for the toner has become even higher, and a better image forming apparatus in actuality is not realized if an improvement of the toner performance cannot be achieved.
For example, a contact developing device adopting a one-component contact developing system is widely prevalent recently, especially for a color device, in a developing step of the electrostatic latent image on the photosensitive member. In the contact developing device for carrying out the development of the latent image, a toner layer on a toner bearing member is brought into contact with a surface of the photosensitive member, and the surface of the photosensitive member and a surface of the toner bearing member moving mutually. Further, a transfer device for electrostatically transferring the toner image on the electrostatic latent image bearing member or an intermediate transferring member to the transfer material, in many cases, employs a contact transfer device in the contact transfer device, a roll transferring member is brought in contact with the electrostatic latent image bearing member or the intermediate transferring member through the transfer material, that is, contact transfer, in view of miniaturization of the image forming apparatus, prevention of ozone generation, or the like.
Controlling a particle shape of the toner to a sphere, for such a contact developing device or a contact transfer device, is effective for improving developability, transferability, and in addition, resistance to mechanical stress received from those devices. However, at the same time, small specific surface area and volume of the spherical toner particles resulted in more than anticipated effects of dispersibility of the colorant inside the toner particles on developability, transferability, and in addition, matching with the image forming apparatus.
Such a phenomenon tends to occur easily with a black toner employing carbon black having a specific surface area larger than those of other colorants and having conductivity. The phenomenon becomes conspicuous particularly with a toner produced through polymerization.
On the other hand, a heat roller-type heat fixing means is widely used as a fixing device for fixing the toner image. The heat roller-type fixing means is provided with a heat roller as a rotary heating member and a pressure roller as a rotary pressing member (hereinafter, both rollers are collectively referred to as “fixing roller”). The toner was desired to express high sharp-melt property during heating thereof alone with miniaturization, speeding up, and power saving of the fixing device. Further, such a toner excels not only in low-temperature fixability, but also in color-mixing property during full-color image forming, and thus, color reproduction range of a fixed image to be obtained could be broadened.
However, such a toner expressing the sharp-melt property generally has high affinity with the fixing roller and tends to cause an offset phenomenon easily, which is a phenomenon of the toner to transfer to a surface of the fixing roller during fixing. The offset phenomenon occurs conspicuously, in particular, during color image forming when a plurality of toner layers form on the transfer material.
Correspondingly, the surface of the fixing roller is coated with a thin film composed of an offset preventing liquid for preventing the offset phenomenon. However, such a method results in adverse effects such as causing upsizing or complication of the fixing device, impairing solid attachment of the fixed image caused by adhesion of the offset preventing liquid, and impairing transparency of a transparency film used for an overhead projector for presentations.
Incidentally, the transfer material used for the image forming apparatus has also been diversified. A type of paper used as the transfer material, for example, not only differs in weight capacity but also varies in materials or content of raw materials or fillers under the present situation. A recycled paper employing a recycled pulp obtained by deinking the paper once used has been widely used recently from a view of environmental protection or the like. An amount of the recycled pulp mixed in the recycled paper and the amount of the recycled paper is presumed to increase more hereafter. Quality of the transfer material varies that the transfer material such as the recycled paper contains components which easily detach therefrom or which easily attach to members of the fixing device. An effect of those transfer materials on the fixing device is large, thereby making miniaturization or life extension of the fixing device difficult. For example, a cleaning member for removing the residual toner or the like from fixing or a separating member for preventing wrapping of the transfer material are arranged on a surface of the heat roller. The arrangements have been confirmed to result in: formation of damages or scratches on the surface of the fixing roller by medium-density fibers in paper powder detached from the recycled paper obtained from, particularly, raw materials of medium-density waste paper such as newspapers and magazines; and remarkable reduction of functions of the cleaning member or the separating member. Such phenomena tend to become more critical issues when using a fixing device with a small amount of the offset preventing liquid applied to the fixing roller or using a fixing device without the application of the offset preventing liquid.
Under such circumstances, technological developments regarding the low-temperature fixability and anti-offset property of the toner have become indispensable. In actuality, multiple strategies with an improved binder resin or wax component have been proposed, but behaviors of the colorant in the toner particles during fixing are hardly studied.
The inventors of the present invention have found out through studies that the colorant such as the pigments in the toner particles not only deprives the binder resin of the sharp-melt property, but the colorant itself also behaves as a fixing inhibitor. In addition, the inventors of the present invention have found out that the colorant possesses a function of disturbing migration of the wax component from the toner particles, thereby degrading the low-temperature fixability and the anti-offset property.
Those phenomena tend to occur more easily with the black toner employing carbon black which has a fine primary particle size and is hardly dispersed uniformly in the toner particles than with other colorant. Those phenomena become conspicuous particularly with a toner produced by polymerization.
The black toner is not only important in office use for reproducing text images, but is also frequently used in graphic images. Here, even finer developability and better low-temperature fixability are demanded for the former case because the amount of the toner used is small for the toner image formed on the transfer material. On the other hand, excellent transferability and anti-offset property are demanded for the latter case because the amount of the toner used, including other chromatic colors, is large for the toner image formed on the transfer material. Therefore, the black toner must achieve satisfactory fixability in a wide temperature range in addition to further improvement in developability or transferability.
Various techniques have been disclosed so far for improving the dispersibility of the carbon black in the toner particles.
For example, JP 2000-352844 A discloses that combining carbon black of a fine particle size and a specific azo metal compound in the presence of a wax component reduces cohesion of the carbon black in the toner particles and liberation from the toner particles. The document also discloses that adaptation of the combination is possible also for a toner produced through polymerization.
However, the specific azo metal compound used in the toner generally expresses pigment-like property, and thus, the azo metal compound had to be treated by adding high shearing force under specific conditions to function as a dispersant. Therefore, dispersibility of the azo metal compound had a limit, and further improvement had been desired regarding fixability, in particular.
Further, JP 05-070511 A discloses a method of producing a toner of a fine particle size through suspension polymerization process using Ti phthalocyanine or soluble Cu phthalocyanine as a dispersant aid for the carbon black.
The toner have improved coloring power and chargeability to some level, but nothing was considered on environmental stability or matching with an image forming apparatus. Further, as the inventors of the present invention have studied, in the case any of the above phthalocyanine compound is used, the dispersibility of carbon black or a polymerizable monomer composition are controlled by functional groups directly bonded to phthalocyanine rings, thereby not providing sufficient dispersion stability. The inventors of the present invention have found out that production of a toner through polymerization process provokes phenomena such as re-aggregation of the carbon black or migration thereof to a surface of the toner particles along with proceeding of a polymerization reaction of the polymerizable monomer.
On the other hand, JP 11-327208 A discloses a technique of applying a charge control resin comprising an acrylamide monomer having a sulfonic group as a component to a toner produced through polymerization.
Those toners are capable of forming a full-color image expressing satisfactory coloring power. However, the inventors of the present invention have found out through studies that further improvement of a dispersion state of a colorant in toner particles have been required. In other words, dispersion stability is not sufficient because improvement in the dispersibility of the colorant is attempted only with the charge control resin. Therefore, the colorant satisfactorily dispersed in a preparation stage of a polymerizable monomer composition provokes phenomena such as re-aggregation of the colorant or migration of the colorant to the surface of the toner particles. Such a toner has not yet further improved in low-temperature fixability or matching with an image forming apparatus.
Further, not much was considered on effects of the colorant on a contact developing device, a contact transfer device, or the like with respect to any of the toners exemplified above. Further, nothing was considered on: residue of aromatic amine derived from raw materials of a colorant; case of using a recycled paper as a transfer material having more than 70% of recycled pulp in mixing ratio; case of forming a color image requiring fixing of a plurality of toner layers formed on a transfer material at once; and performance in a case of using a fixing device with a small amount of an offset preventing liquid applied to a fixing roller or using a fixing device without the application of the offset preventing liquid.
Namely, a system design of the image forming apparatus using the contact developing means, the contact transfer means, or the heat pressure fixing means as described above is not yet sufficient in overall strategies embracing the colorant used for the toner.